Measurement of the Multiplication of a Spent Fuel Assembly with the Differential Die-away Method Within the Scope of the Next Generation Safeguards Initiative Spent

As part of the Next Generation Safeguards Initiative (NGSI)spent fuel project, researchers evaluated the ability of fourteendifferent non-destructive assay techniques to determine elementalplutonium content in a spent fuel assembly (SFA); the DifferentialDie-away (DDA) technique is one of these techniques.1DDA uses short neutron pulses generated by an external neutrongenerator to actively interrogate the material within a SFA.The measured response is then predominantly prompt neutronsfrom induced fission of 235U, 239Pu, and 241Pu detected by 3Hetubes positioned around the assayed SFA. Due to its rich andcomplex dynamic response, the neutron-generator-driven DDAtechnique is considered a potential candidate for high-accuracyapplications (e.g., in nuclear fuel reprocessing plants or geologicrepositories). In this paper, we use MCNPX simulations to investigateseveral methods to directly measure the multiplication(M) of the SFA, which is a crucial characteristic ultimately reflectingits initial enrichment (IE), burn-up (BU), and coolingtime (CT). The results are based on the analysis of a simulatedresponse of the DDA instrument to the active interrogation ofsixty-four SFA’s from NGSI Spent-Fuel Library 1 (SFL1).2 Ourfindings indicate that the multiplication can be determined bythree different approaches: (1) The ratio of count rates betweenthe detectors nearest to and farthest from the neutron generator(i.e., back-to-front ratio) can be used to measure multiplicationin the time domain soon after the interrogating pulse (0-200µs);(2) the die-away time constant is a suitable measure of multiplicationonly after approximately 500µs; and (3) the total number ofprompt fission neutrons detected within the first millisecond afterthe neutron pulse scales with multiplication, although similarinformation may be obtained by detecting neutrons in a reducedtime domain between 100 and 200 µs, which is generally lessprone to electronics saturation directly following the interrogatingneutron pulse.